One of the most basic aspects of the present invention relates to a piezoelectric drive of the general type such as shown in U.S. Pat. Nos. 4,400,641, 4,453,103, 4,959,580 and 6,469,420 (the disclosures of which are hereby incorporated by reference herein). Such drives have a rotor, stator, a piezoelectric oscillator, and elastic pushers acting between the rotor and oscillator to move the rotor. The invention also relates to component parts, particularly rotors and/or pushers, of such piezoelectric drives, and the use of such drives in the construction of analog instruments, and in the construction of a wide variety of other mechanical devices apart from analog instruments.
The piezoelectric drives of the above-identified patents effect rotation of the rotors by generation of longitudinal ultrasonic fluctuations in the elastic pushers. As a result of these fluctuations, the ends of the pushers are pressed onto the rotor's friction surface, and the rotor and pushers are caused to rotate with respect to each other.
Conventional rotor friction surfaces are made of hard materials that are capable of resisting the influence of the powerful ultrasonic field created at the rotor-engaging end of each pusher. Typical materials are steel, aluminum oxide, and zirconium oxide. However, such conventional hard materials used for rotors, especially when used with steel pushers, are very abrasive. The abrasive action between the pushers and rotors causes rapid wear of the rotor and pusher materials, fine particles of the rotor and pushers breaking off. When the broken-off steel particles are exposed to air they typically oxidize, e.g. turning into a hard abrasive powder of iron oxide. This means that conventional piezoelectric drives have an effective life of only about 100-1000 hours of actual operation, too short for many applications (including, but not limited to, analog instruments).
The fine particles of iron oxide cause other problems. They stick to the working surface of the rotor, and penetrate it. The combination of wear and penetration causes a non-uniform rotor surface, resulting in an inexact degree of rotation of the rotor for each actuation of the piezoelectric oscillator. This limits applicability of conventional piezoelectric drives in applications (including, but not limited to, analog instruments and servo systems) where drive precision is necessary.
Yet another weakness inherent in rotors made from materials like steel, aluminum oxide, and zirconium oxide, is their high mechanical quality factor. Every resonance system has its own mechanical quality factor, represented by the formula Q=Fr/(F2−F1). Fr is the frequency of the system at maximum resonant amplitude, and F1 and F2 are the frequencies before and after the maximum resonant amplitude, respectively, at the positions defined by the maximum amplitude divided by the square root of two. The value of Q for steel is about 2000, and for conventional piezoelectric ceramics about 1000. Such a high Q contributes to the creation of parasitic ultrasonic fluctuations, which in turn lead to the dampening of energy transmitted by the pushers to the rotors, reducing the overall performance of the drive. In addition, materials with a high Q factor typically require a significant amount of precision machining, and therefore are costly, especially when compared with injection molded parts.
The use of hard wear-resistant plastic materials, such as a paper-based laminate, for the outer ring of a rotor or for pushers (see U.S. Pat. No. 4,453,103, col. 10, lines 18-20 and col. 12, lines 25-30) does not solve all of the problems set forth above, or have all of the advantages of the invention. Also, the use of a material with a low Q factor, per se, such as conventional amorphous thermoplastic polymers, also causes significant problems. The use of amorphous thermoplastics such as poliatsetals and polystyrenes with firm filler materials with similar properties, in the construction of rotors is typically unsuccessful. These materials have insufficient thermal stability at the frictional surface of the rotor, and start to melt or lose strength when exposed to the powerful ultrasonic fields typically created in piezoelectric drives. Therefore they too have short lives, and introduce inaccuracies into incremental movements over time.
EPO Patent Application 0 932 208 A1 proposes making a piezoelectric motor with an injection molded rotor of semi-crystalline thermoplastic polymer with filler, and a stator and holder of polyamide with a 20-30% glass fiber content and also injection molded. The material of the plates (“Lamellen”) 3 does not appears to be specified, but the plates are apparently shown integral with the oscillator 2, and the oscillator is of piezo-ceramic material. The preferred polymeric material for the rotor is polyarylamide; although the amount of filler is not specified, the properties of the rotor material set forth correspond to a commercial polyarylamide with about 60% filler. While the rotor of the EPO reference is likely to be highly advantageous, any advantages associated with the plates 3 are speculative at best, and if the plates are of piezo-ceramic material that would be a drawback for long life applications that are encompassed according to some aspects of the present invention. Also, there may be some circumstances where it is desirable to make only the friction surface of the rotor of semi-crystalline thermoplastic polymer with filler, rather than make the entire rotor of that material as an integral injection molded part as is required in the EPO publication.
According to one aspect of the present invention, a piezoelectric drive, and its components (particularly rotors and/or pushers thereof), are provided that overcome the problems inherent in the conventional prior art discussed above, and can be advantageous compared to the structure of the EPO reference. The piezoelectric drives, and components, according to the invention have significantly reduced wear of the pushers and rotors, resulting in longer life. For example, a piezoelectric drive according to the invention having a rotor according to the invention can be expected to have an effective life of over about 6000 hours of actual operation, e.g. about 7000 hours. If pushers according to the invention are also employed, the effective life can be expected to be over about 9000 hours of actual operation, e.g. about 10,000 hours.
In addition to longer life, the piezoelectric drives, and components, according to the invention have a more uniform and stable rotation of the rotor (all other factors being equal), resulting in higher accuracy of positioning. This thus expands the market for piezoelectric drives to precision devices (including, but not limited to, analog instruments, and servo systems). Also, according to the invention, the above advantages are achieved while simultaneously reducing the cost of manufacture of piezoelectric drives using steel or ceramic rotors and pushers.
The advantages of one aspect of the invention are basically accomplished by using as the rotor friction surface material, and also the material for the pushers, one that has a low mechanical quality factor (e.g. Q is less than about 500, typically less than about 200, preferably less than about 100, and most preferably about 30-50). Yet the material has high strength, reduced hardness (compared to steel and ceramics), and is able to withstand the strong ultrasonic fields of piezoelectric devices without melting or losing a significant amount of strength. For example, the strength of the material [expressed as ultimate tensile strength, although other measures of strength may be employed] is preferably at least about 140 MPa (more desirably at least about 205 MPa, and most desirably at least about 225 MPa).
One highly desirable class of materials for use as the rotors (whether just simply the friction surface thereof, or the entire rotor body injection molded in one piece) and pushers according to the invention comprises semi-crystalline thermoplastic polymer with filler. While a number of such polymers may be suitable (e.g. polyphenylene sulfide, nylon 6,6, or subsequently developed or modified polymers), the presently preferred polymer is polyarylamide. This polymer has a glass transition temperature of about 85 degrees C. (and therefore remarkable rigidity for a polymeric material). It also has high strength (e.g. with about 50% filler about 255 MPa ultimate tensile strength—higher than that of many metals—and about 380 MPa flexural yield strength), low creep, an excellent surface finish, and a Q factor of about 30-50. Components, or parts of final components (such as the friction surface of a rotor), made from polyarylamide are readily produced by injection molding, making them relatively inexpensive as well as mechanically desirable.
According to one aspect of the present invention there is provided an elastic pusher for a piezoelectric drive, comprising: an elastic pusher body comprising, or consisting essentially of, semi-crystalline thermoplastic polymer and filler, which does not lose a significant amount of strength or start to melt when exposed to ultrasonic fields typically used in piezoelectric drives, the body configured and dimensioned to cooperate with a stator, rotor, and piezoelectric oscillator in a piezoelectric drive. For example, the pusher body may consist essentially of about 30-60% (preferably about 40-50%) by weight semi-crystalline thermoplastic polymer (e.g. polyarylamide) and about 40-70% (preferably about 50-60%) by weight filler. The pusher may be integral with a ring and a plurality of other pushers (all extending radially from the ring) of the same material. The ring is dimensioned and configured to fit on a piezoelectric oscillator. The ring may be press fit onto, adhesively attached to, or molded onto, the piezoelectric oscillator.
If desired, the pusher body may comprise a plurality of plates connected together, at least some of the plates having oriented fibers in the dimension of elongation thereof. The multi-plate pusher may be connected to a holder along with a plurality of like pushers, by crimping, adhesive, soldering, welding, etc.
Most desirably, the pusher body is injection molded and elongated in a dimension, and comprises a polymer, and at least 10% by weight fibers substantially oriented in the dimension of elongation of the pusher body (as well as possibly other filler), e.g. up to 60% of the pusher weight could be oriented fibers. The pusher body material also preferably has an ultimate tensile strength of at least about 140 MPa (more desirably at least about 205 MPa, and most desirably at least about 225 MPa). Also, the elastic pusher preferably has an effective life of at least about 9000 hours (e.g. about 10,000 hours) of operation in a piezoelectric drive when cooperating with a rotor body having a friction surface also of semi-crystalline thermoplastic polymer and filler which does not lose a significant amount of strength or start to melt when exposed to ultrasonic fields typically used in piezoelectric drives. The preferred thermoplastic polymer is polyarylamide, and the filler may be selected from the group consisting primarily of particles of glass, fiberglass, particles or fibers of oxide ceramics, metals, carbon, or graphite, and combinations thereof.
According to another aspect of the present invention there is provided a rotor for a piezoelectric drive, comprising: a rotor body having a friction surface of semi-crystalline thermoplastic polymer and filler, which does not lose a significant amount of strength or start to melt when exposed to ultrasonic fields typically used in piezoelectric drives, the body and friction surface thereof configured and dimensioned to cooperate with a stator, pushers, and piezoelectric oscillator in a piezoelectric drive, with the pushers operatively engaging the friction surface of the rotor. While for some purposes the body may be essentially one piece, e.g. injection molded of a semi-crystalline thermoplastic polymer and filler, typically the friction surface can be formed of a different material than the rest of the body. For example, the friction surface can be injection molded as a ring (e.g. of about 45% polyarylamide and 55% filler) which is fit onto or into and attached to a core element to form the body. The core element may be of a cheaper material (e.g. cheaper plastic, such as polyamide) than polyarylamide, and may also be injection molded. The ring may be press fit onto, adhesively attached to, welded to, and/or otherwise attached to the core element.
Instead of being of necessarily cheaper material, the core element of the rotor may be of metal (such as a molded aluminum alloy), or other material that is an excellent conductor of heat. In this way the core element of the rotor may provide a heat sink for the often high heat levels created by the interaction of the rotor friction surface and pusher ends during operation of the piezoelectric drive.
Regardless of the material of the core element, the friction material ring of the rotor may be located in a wide variety of positions with respect to the core element, including as an interior surface, or exterior surface.
The body friction surface may comprise or consist essentially of semi-crystalline thermoplastic polymer and filler. For example, the body friction surface may consist essentially of about 30-60% (preferably about 40-50%) by weight semi-crystalline thermoplastic polymer and about 40-70% (preferably about 50-60%) by weight filler. For example, the body, or the friction surface of the body as a ring, may be injection molded, and the semi-crystalline thermoplastic polymer may comprise polyarylamide, and the filler may be selected from the group consisting primarily particles of glass, fiberglass, particles or fibers of oxide ceramics, metals, carbon, or graphite, and combinations thereof. The rotor body friction surface may have an ultimate tensile strength of at least about 140 MPa (preferably at least about 205 MPa, e. g. at least about 225 MPa), and the rotor preferably has an effective life of at least about 6000 hours of operation in a piezoelectric drive.
The invention also encompasses a piezoelectric drive comprising: A first stator. A first piezoelectric oscillator. A first rotor with a friction surface of semi-crystalline thermoplastic polymer with filler, which does not lose a significant amount of strength or start to melt when exposed to ultrasonic fields typically used in piezoelectric drives. A driven element (typically a shaft) operatively connected to the first rotor. And a plurality of elastic pushers cooperating between the first rotor friction surface and first oscillator, to move the rotor in a first direction with respect to the stator upon actuation of the piezoelectric oscillator, the pushers comprising semi-crystalline thermoplastic polymer with filler, which does not lose a significant amount of strength or start to melt when exposed to ultrasonic fields typically used in piezoelectric drives.
Preferably, the material of the pushers has a mechanical quality factor of less than about 200 (e.g. about 30-50), and has an ultimate tensile strength of at least about 140 Mpa, e.g. is about 30-60% polyarylamide with about 40-70% filler.
The piezoelectric drive may further comprise: A second piezoelectric oscillator. A second rotor with a friction surface. And a plurality of elastic pushers cooperating between the second rotor and second oscillator; and wherein the second rotor friction surface and the plurality of pushers, comprise semi-crystalline thermoplastic polymer with filler, which does not lose a significant amount of strength or start to melt when exposed to ultrasonic fields typically used in piezoelectric drives. The first and second rotors are operatively connected so that actuation of the first and second piezoelectric oscillators, respectively, moves the driven element (shaft) in opposite first and second directions (e.g. clockwise, and counterclockwise).
In the piezoelectric drive described above, the rotors may be operatively connected to a pointer of an analog instrument, the pointer mounted on the shaft. The drive may then further comprise an angular position sensor which operatively senses the position of the shaft, and an environmental condition sensor operatively connected to the angular position sensor, and operatively connected to the piezoelectric oscillators to control operation thereof to move the pointer in the first or second directions in response to a sensed environmental condition.
Alternatively, the first rotor is operatively connected to a driven mechanical element excluding a pointer (such as a point of purchase display, as in U.S. Pat. No. 6,502,338, or a main component of a servo system). Or, the first rotor may be operatively connected to a time measuring hand.
Also, the first rotor may be injection molded (completely, or just the friction surface thereof) of about 30-60% by weight semi-crystalline thermoplastic polymer (e.g. polyarylamide) and about 40-70% by weight filler (e.g. selected from the group consisting primarily of glass particles, fiberglass, and particles or fibers of oxide ceramics, metals, carbon, or graphite, and combinations thereof).
According to still another aspect of the invention, there is provided an instrument comprising: a piezoelectric drive comprising: a first stator, a first piezoelectric oscillator, a first rotor, and a first plurality of elastic pushers cooperating between the first rotor and first oscillator; and a second piezoelectric oscillator, a second rotor, and a second plurality of elastic pushers cooperating between the second rotor and second piezoelectric oscillator. A shaft operatively connected to the first and second rotors. Wherein the first and second rotors are operatively connected so that actuation of the first and second piezoelectric oscillators, respectively, moves the shaft in opposite first and second directions. And an environmental condition sensor operatively connected to the piezoelectric oscillators to control operation thereof to move the shaft in the first or second directions in response to a sensed environmental condition.
The instrument preferably comprises an analog instrument (although the shaft may be connected to a conventional device that generates a digital display); and further comprises a pointer mounted to the shaft for movement therewith, and a scale which the pointer moves relative to.
The environmental condition sensor may sense one or more environmental conditions, such as speed, temperature, atmospheric or other pressure, altitude, a concentration of one or more particular agents in a gaseous or liquid medium, wind velocity, solar radiation, weight, radioactive decay, moisture, water depth, pH, salinity etc. The instrument also preferably includes an angular position sensor which operatively senses the position of the shaft; and is also operatively connected to the environmental condition sensor. In the instrument described, the first and second rotor friction surfaces (either the entire rotor, or just the friction surface) and/or the pushers may be injection molded of about 30-60% by weight semi-crystalline thermoplastic polymer, with about 4070% filler, and have an effective life of at least about 6000 hours of operation, preferably at least about 9000 hours of operation if both the pushers and rotor friction surfaces are of the specified material.
It is the primary object of the present invention to provide an improved, long-life, accurate, and relatively inexpensive, piezoelectric drive and components thereof, and instruments or devices made therefrom. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.